a. Field of the Invention
The present invention relates to an optical low pass filter to be used in optical systems which use solid-state image pickup devices for obtaining output information by distributedly sampling image information, and more specifically to an optical low pass filter suited for use in optical systems which use one-dimensional solid-state image pickup devices, or, so-called line sensors.
b. Description of the Prior Art
In an optical system which uses a solid-state image pickup device such as a CCD for obtaining output information by distributedly sampling image information, signals picked up and output by the solid-state image pickup device contain spurious signals which are not originally from an object when the object comprises components having spatial frequencies higher than the sampling frequency of the solid-state image pickup device. In other words, components which have spatial frequencies higher than the Nyquist frequency and cannot be picked up by the solid-state image pickup device, are output as moire, thereby constituting a cause for remarkable degradation of image quality.
Accordingly, the optical system described above conventionally adopts a method to use, as a portion of its image pickup optical system, an optical low pass filter for limiting the components having the high spatial frequencies of the object which are to be input into the solid-state image pickup device. This method is roughly divided into a method which obtains a low pass effect by separating an image point in a direction perpendicular to an optical axis with a birefringent uniaxial crystal such as quartz or a phase filter utilizing a diffraction phenomenon and another method which obtains a low pass effect by defocusing a lens component so as to cut off the components having the high frequencies.
However, the former method increases the manufacturing cost of the optical system since quartz is very expensive and the phase filter requires high manufacturing precision. Further, the latter method makes it difficult to obtain sufficient imaging performance since components having low frequencies are remarkably degraded when the components that have high frequencies are cut off with the defocused lens component.
Furthermore, a general low pass filter is configured based on a premise that it is to be used with a two-dimensional image pickup device and has a composition which is not always optimum for a one-dimensional image pickup device, or, the so-called line sensor.